Conventionally, data communication for wirelessly transmitting various data through FSK radio communication using an FSK (frequency shift keying) signal is widely used as a system of data communication. The FSK signal is one of FM (frequency modulation) signals.
In the data communication through FSK radio communication, when data is transmitted, a high-frequency signal is transmitted as a radio wave, the high-frequency signal having been frequency-modulated by frequency-shifting a carrier so as to correspond to 1 and 0 of a digital signal of the data. When the high-frequency signal is received by an FSK receiver and the transmitted data is demodulated, the frequency components of a demodulation baseband signal having been voltage-converted by an F-V converter circuit are compared with each other by a comparator to determine a digital value, and the original data is obtained based on the digital value.
In the frequency modulation system, between a transmit side and a receive side, a frequency offset occurs due to a frequency generation error on a quartz radiator, a local oscillator (hereinafter, referred to as PLL), and the like of the transmit side and receive side. The frequency offset becomes a DC offset component when frequency-voltage conversion is performed to obtain a demodulation baseband signal.
In conventional FSK receivers, FSK demodulation is performed in an analog manner, and thus a frequency offset is equivalent to a fluctuation in the reference voltage of a comparator for data decision. Further, due to a frequency offset, the frequency of a carrier is shifted close to the cutoff frequency of a filter when the carrier passes through the filer. Thus, particularly in the case of communications in a narrow transmission band, the attenuation of the filter exercises considerable influence. These factors have seriously adverse effects on the receiving characteristic of the FSK receivers.
As described above, in the conventional FSK receivers (for example, Japanese Patent Laid-Open No. 2000-349840), FSK demodulation is performed in an analog manner. In this case, a demodulation baseband signal having undergone frequency-voltage conversion is extracted by C coupling and a DC offset component on the demodulation baseband signal is removed, so that a demodulation error caused by a DC offset is corrected and the adverse effects on the receiving characteristic are avoided.
Further, a difference is obtained between the signal where the DC offset component on the demodulation baseband signal is removed by C coupling and a demodulation baseband signal with a DC offset before C coupling, so that the DC offset component is detected on the demodulation baseband signal having undergone frequency-voltage conversion. The reference voltage in a comparator for data decision is corrected according to an offset amount and a frequency offset is equally corrected.
However, in the conventional method of correcting a frequency offset, all circuits including detectors and correcting units conventionally have analog configurations in the FSK receivers. In this case, there are many variations in operating characteristics among the circuits, the accuracy of detecting a DC offset component from a demodulation baseband signal is reduced by noise, and thus a DC offset cannot be accurately detected or corrected. Consequently, operations become susceptible to noise and become less stable in a data communication system through FSK radio communication.
Once a malfunction occurs due to noise, the circuits may enter an oscillation state. Thus, the power consumption of an overall system increases, the data communication system becomes less stable, and the reliability of data communications seriously decreases.